The invention relates generally to power-driven conveyors and, more particularly, to conveyors using conveyor belts with article-supporting rollers rotatable in the direction of belt travel to accumulate and selectively release accumulated articles and methods for their operation.
Conveyors are used in industrial applications to transport articles from one processing station to another. Often a continuously moving conveyor belt is used to deliver articles to a downstream processing station. As soon as the supply of articles exceeds the handling capacity of the downstream processing station, the articles begin to back up at a stopping mechanism blocking the conveyor at a position upstream of the processing station. Friction between the conveying surface of the moving belt and the backed-up, or accumulated, articles causes the articles to push against each other. The pressure exerted against the article at the front of the group of accumulated articles is called back line pressure. As more articles back up, the back line pressure increases. Back line pressure can cause damage to the articles, excessively load the conveyor belt and its drive components, and accelerate belt wear. Zone accumulation, in which a series of separately driven conveyors can be stopped and started depending on the downstream demand and upstream supply of articles, is used to eliminate back line pressure. But zone accumulation requires multiple drives and sensors, which can be costly, and, between consecutive conveying zones, transfers, at which articles are susceptible to tipping.
Roller-top belts with freely rotatable rollers in rolling contact with the undersides of conveyed articles are often used to reduce friction and lower back line pressure. The roller-top belts are typically followed by an indexing conveyor with a high-friction conveying surface that stops to serve as a stop mechanism and allow upstream articles to accumulate with low back line pressure atop the rollers of the roller-top belt and starts to release articles for delivery downstream. Like zone accumulation, this arrangement requires separately driven conveyors and suffers from some of the other shortcomings.
Thus, there is a need for a conveyor that can accumulate and release conveyed articles without the shortcomings of zone accumulators and separate accumulation and indexing conveyors.